1. Field of the Invention
The present invention relates to a methodology and apparatus for driving a stepping motor and in particular relates to the means and ways in which a stepping motor may be employed in a digital circuit to drive the paper feed on a hard copy printer.
2. Prior Art
Hard copy printers for digital circuitry in computers utilize paper feeds employing various types of stepping motors. The stepping motor is coupled through a conventional mechanical power train which ultimately drives a sprocketed wheel. The sprocketed wheel is coupled to the paper fed through the printer by means of holes provided in the edges of the paper which are engaged by the sprocketed wheel.
When it is desired that the paper be fed through the digital printer, the stepping motor is cycled through a series of steps. Since the amount of paper fed must be accurate and uniform in order to create a reliable printout, some means must be employed to accurately stop the stepping motor at a predetermined position. Typically, prior art stepping motors using phased stator coils or poles employed a methodology and circuitry wherein the plurality of sets of phased stator coils were driven sequentially. The set of phased coils energized would create a discretely rotating configuration around the sets of stator coils which would be followed by the rotor of the stepping motor.
When the predetermined position had been reached, the next to the last set of coils which had been energized would be re-energized in order to create a braking effect upon the rotating rotor. After the momentum of the rotor had been substantially decreased by the reversed biased coils, the reverse biased coils would be appropriately de-energized and the last set of coils re-energized in order the fix or urge the rotor into the predetermined position.
As a result of the methodology and apparatus used, two additional cycles would have to be added to the normal driving sequence used in the stepping motor, namely, one phase to brake the rotor and a second phase to fix or urge the rotor in the predetermined position. In addition, the position of the rotor would tend to ring about the predetermined end position since the rotor was usually stopped hard in its end position. Finally, the torque supplied by the motor would discretely follow the energization of the separate stator coils with the result that the torques applied by the motor through the sprocket wheels to the paper would tend to be impulsive and jerky. In some cases, the sprockets would substantially deform, tear, or stretch the sprocket holes provided in the edges of the paper with the result that there would be a paper feed failure or a certain degree of misalignment in the printed, hard copy lines. The present invention discloses a methodology and apparatus for overcoming each of these shortcomings of the prior art.